Zipline

The project uses the fraud proof based protocol from Optimism Cannon to provably execute the Eth2 light client protocol on another EVM chain. This allows the target EVM chain to stay in sync with Eth2 finalized blocks.

An off-chain relayer submits a finalized block along with a sync committee update. The update contains signatures from the current sync committee plus the proofs to derive the next committee.

Instead of executing the signature verification on-chain (which would be prohibitively expensive) they are executed off-chain and a proof-of-execution submitted on-chain. If the relayer commits fraud this can be proven by any honest observer by submitting a challenge during a challenge period.

Once this period has elapsed the update is assumed to be valid. This process is repeated once per sync period (27 hours) to keep the target chain in sync with the Eth2 state. Consumers can safely use the Eth2 block roots to prove state or transaction inclusion which could be used to build a trustless token bridge.

The Eth2 light client code to validate signatures and sync committee was adapted from the Snowbridge Eth2->Substrate bridge. The verification code is compiled to the MIPS instruction set which allows the execution trace to be proven using Optimism Cannon.

We also modified the Optimim Cannon codebase to operate as a bridge rather than a rollup. It keeps track of the pending eth2 block hash and the prior block hashes that have reached finality after their challenge period has elapsed. Another difference from Optimism is that the required data to perform the provable execution is not available on-chain. The contract was also modified such that the relayer is required to submit all data required to verify the proof on-chain each period.